HTTPbis Working Group R. Fielding, Ed.
Internet-Draft Adobe
Obsoletes: 2616 (if approved) J. Gettys
Intended status: Standards Track Alcatel-Lucent
Expires: September 15, 2011 J. Mogul
HP
H. Frystyk
Microsoft
L. Masinter
Adobe
P. Leach
Microsoft
T. Berners-Lee
W3C/MIT
Y. Lafon, Ed.
W3C
J. Reschke, Ed.
greenbytes
March 14, 2011
HTTP/1.1, part 5: Range Requests and Partial Responsesdraft-ietf-httpbis-p5-range-13
Abstract
The Hypertext Transfer Protocol (HTTP) is an application-level
protocol for distributed, collaborative, hypermedia information
systems. HTTP has been in use by the World Wide Web global
information initiative since 1990. This document is Part 5 of the
seven-part specification that defines the protocol referred to as
"HTTP/1.1" and, taken together, obsoletes RFC 2616. Part 5 defines
range-specific requests and the rules for constructing and combining
responses to those requests.
Editorial Note (To be removed by RFC Editor)
Discussion of this draft should take place on the HTTPBIS working
group mailing list (ietf-http-wg@w3.org). The current issues list is
at <http://tools.ietf.org/wg/httpbis/trac/report/3> and related
documents (including fancy diffs) can be found at
<http://tools.ietf.org/wg/httpbis/>.
The changes in this draft are summarized in Appendix D.14.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
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Internet-Draft HTTP/1.1, Part 5 March 2011
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 15, 2011.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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the Trust Legal Provisions and are provided without warranty as
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Contributions published or made publicly available before November
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 41.1. Requirements . . . . . . . . . . . . . . . . . . . . . . . 41.2. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 41.2.1. Core Rules . . . . . . . . . . . . . . . . . . . . . . 5
1.2.2. ABNF Rules defined in other Parts of the
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Internet-Draft HTTP/1.1, Part 5 March 20111. Introduction
HTTP clients often encounter interrupted data transfers as a result
of cancelled requests or dropped connections. When a cache has
stored a partial representation, it is desirable to request the
remainder of that representation in a subsequent request rather than
transfer the entire representation. There are also a number of Web
applications that benefit from being able to request only a subset of
a larger representation, such as a single page of a very large
document or only part of an image to be rendered by a device with
limited local storage.
This document defines HTTP/1.1 range requests, partial responses, and
the multipart/byteranges media type. The protocol for range requests
is an OPTIONAL feature of HTTP, designed so resources or recipients
that do not implement this feature can respond as if it is a normal
GET request without impacting interoperability. Partial responses
are indicated by a distinct status code to not be mistaken for full
responses by intermediate caches that might not implement the
feature.
Although the HTTP range request mechanism is designed to allow for
extensible range types, this specification only defines requests for
byte ranges.
1.1. Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
An implementation is not compliant if it fails to satisfy one or more
of the "MUST" or "REQUIRED" level requirements for the protocols it
implements. An implementation that satisfies all the "MUST" or
"REQUIRED" level and all the "SHOULD" level requirements for its
protocols is said to be "unconditionally compliant"; one that
satisfies all the "MUST" level requirements but not all the "SHOULD"
level requirements for its protocols is said to be "conditionally
compliant".
1.2. Syntax Notation
This specification uses the ABNF syntax defined in Section 1.2 of
[Part1] (which extends the syntax defined in [RFC5234] with a list
rule). Appendix C shows the collected ABNF, with the list rule
expanded.
The following core rules are included by reference, as defined in
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Internet-Draft HTTP/1.1, Part 5 March 2011[RFC5234], Appendix B.1: ALPHA (letters), CR (carriage return), CRLF
(CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double quote),
HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit
sequence of data), SP (space), VCHAR (any visible USASCII character),
and WSP (whitespace).
1.2.1. Core Rules
The core rules below are defined in Section 1.2.2 of [Part1]:
token = <token, defined in [Part1], Section 1.2.2>
OWS = <OWS, defined in [Part1], Section 1.2.2>
1.2.2. ABNF Rules defined in other Parts of the Specification
The ABNF rules below are defined in other parts:
HTTP-date = <HTTP-date, defined in [Part1], Section 6.1>
entity-tag = <entity-tag, defined in [Part4], Section 2>
2. Range Units
HTTP/1.1 allows a client to request that only part (a range of) the
representation be included within the response. HTTP/1.1 uses range
units in the Range (Section 5.4) and Content-Range (Section 5.2)
header fields. A representation can be broken down into subranges
according to various structural units.
range-unit = bytes-unit / other-range-unit
bytes-unit = "bytes"
other-range-unit = token
HTTP/1.1 has been designed to allow implementations of applications
that do not depend on knowledge of ranges. The only range unit
defined by HTTP/1.1 is "bytes". Additional specifiers can be defined
as described in Section 2.1.
If a range unit is not understood in a request, a server MUST ignore
the whole Range header field (Section 5.4). If a range unit is not
understood in a response, an intermediary SHOULD pass the response to
the client; a client MUST fail.
2.1. Range Specifier Registry
The HTTP Ranger Specifier Registry defines the name space for the
range specifier names.
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Registrations MUST include the following fields:
o Name
o Description
o Pointer to specification text
Values to be added to this name space are subject to IETF review
([RFC5226], Section 4.1).
The registry itself is maintained at
<http://www.iana.org/assignments/http-range-specifiers>.
3. Status Code Definitions3.1. 206 Partial Content
The server has fulfilled the partial GET request for the resource.
The request MUST have included a Range header field (Section 5.4)
indicating the desired range, and MAY have included an If-Range
header field (Section 5.3) to make the request conditional.
The response MUST include the following header fields:
o Either a Content-Range header field (Section 5.2) indicating the
range included with this response, or a multipart/byteranges
Content-Type including Content-Range fields for each part. If a
Content-Length header field is present in the response, its value
MUST match the actual number of octets transmitted in the message-
body.
o Date
o Cache-Control, ETag, Expires, Content-Location, Last-Modified,
and/or Vary, if the header field would have been sent in a 200
response to the same request
If the 206 response is the result of an If-Range request, the
response SHOULD NOT include other representation header fields.
Otherwise, the response MUST include all of the representation header
fields that would have been returned with a 200 (OK) response to the
same request.
A cache MUST NOT combine a 206 response with other previously cached
content if the ETag or Last-Modified header fields do not match
exactly, see Section 4.
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A cache that does not support the Range and Content-Range header
fields MUST NOT cache 206 (Partial Content) responses. Furthermore,
if a response uses a range unit that is not understood by the cache,
then it MUST NOT be cached either.
3.2. 416 Requested Range Not Satisfiable
A server SHOULD return a response with this status code if a request
included a Range header field (Section 5.4), and none of the ranges-
specifier values in this field overlap the current extent of the
selected resource, and the request did not include an If-Range header
field (Section 5.3). (For byte-ranges, this means that the first-
byte-pos of all of the byte-range-spec values were greater than the
current length of the selected resource.)
When this status code is returned for a byte-range request, the
response SHOULD include a Content-Range header field specifying the
current length of the representation (see Section 5.2). This
response MUST NOT use the multipart/byteranges content-type.
4. Combining Ranges
A response might transfer only a subrange of a representation, either
because the request included one or more Range specifications, or
because a connection closed prematurely. After several such
transfers, a cache might have received several ranges of the same
representation.
If a cache has a stored non-empty set of subranges for a
representation, and an incoming response transfers another subrange,
the cache MAY combine the new subrange with the existing set if both
the following conditions are met:
o Both the incoming response and the cache entry have a cache
validator.
o The two cache validators match using the strong comparison
function (see Section 4 of [Part4]).
If either requirement is not met, the cache MUST use only the most
recent partial response (based on the Date values transmitted with
every response, and using the incoming response if these values are
equal or missing), and MUST discard the other partial information.
5. Header Field Definitions
This section defines the syntax and semantics of HTTP/1.1 header
fields related to range requests and partial responses.
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Internet-Draft HTTP/1.1, Part 5 March 20115.1. Accept-Ranges
The "Accept-Ranges" header field allows a resource to indicate its
acceptance of range requests.
Accept-Ranges = "Accept-Ranges" ":" OWS Accept-Ranges-v
Accept-Ranges-v = acceptable-ranges
acceptable-ranges = 1#range-unit / "none"
Origin servers that accept byte-range requests MAY send
Accept-Ranges: bytes
but are not required to do so. Clients MAY generate range requests
without having received this header field for the resource involved.
Range units are defined in Section 2.
Servers that do not accept any kind of range request for a resource
MAY send
Accept-Ranges: none
to advise the client not to attempt a range request.
5.2. Content-Range
The "Content-Range" header field is sent with a partial
representation to specify where in the full representation the
payload body is intended to be applied.
Range units are defined in Section 2.
Content-Range = "Content-Range" ":" OWS Content-Range-v
Content-Range-v = content-range-spec
content-range-spec = byte-content-range-spec
/ other-content-range-spec
byte-content-range-spec = bytes-unit SP
byte-range-resp-spec "/"
( instance-length / "*" )
byte-range-resp-spec = (first-byte-pos "-" last-byte-pos)
/ "*"
instance-length = 1*DIGIT
other-content-range-spec = other-range-unit SP
other-range-resp-spec
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other-range-resp-spec = *CHAR
The header field SHOULD indicate the total length of the full
representation, unless this length is unknown or difficult to
determine. The asterisk "*" character means that the instance-length
is unknown at the time when the response was generated.
Unlike byte-ranges-specifier values (see Section 5.4.1), a byte-
range-resp-spec MUST only specify one range, and MUST contain
absolute byte positions for both the first and last byte of the
range.
A byte-content-range-spec with a byte-range-resp-spec whose last-
byte-pos value is less than its first-byte-pos value, or whose
instance-length value is less than or equal to its last-byte-pos
value, is invalid. The recipient of an invalid byte-content-range-
spec MUST ignore it and any content transferred along with it.
In the case of a byte range request: A server sending a response with
status code 416 (Requested range not satisfiable) SHOULD include a
Content-Range field with a byte-range-resp-spec of "*". The
instance-length specifies the current length of the selected
resource. A response with status code 206 (Partial Content) MUST NOT
include a Content-Range field with a byte-range-resp-spec of "*".
Examples of byte-content-range-spec values, assuming that the
representation contains a total of 1234 bytes:
o The first 500 bytes:
bytes 0-499/1234
o The second 500 bytes:
bytes 500-999/1234
o All except for the first 500 bytes:
bytes 500-1233/1234
o The last 500 bytes:
bytes 734-1233/1234
When an HTTP message includes the content of a single range (for
example, a response to a request for a single range, or to a request
for a set of ranges that overlap without any holes), this content is
transmitted with a Content-Range header field, and a Content-Length
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header field showing the number of bytes actually transferred. For
example,
HTTP/1.1 206 Partial Content
Date: Wed, 15 Nov 1995 06:25:24 GMT
Last-Modified: Wed, 15 Nov 1995 04:58:08 GMT
Content-Range: bytes 21010-47021/47022
Content-Length: 26012
Content-Type: image/gif
When an HTTP message includes the content of multiple ranges (for
example, a response to a request for multiple non-overlapping
ranges), these are transmitted as a multipart message. The multipart
media type used for this purpose is "multipart/byteranges" as defined
in Appendix A.
A response to a request for a single range MUST NOT be sent using the
multipart/byteranges media type. A response to a request for
multiple ranges, whose result is a single range, MAY be sent as a
multipart/byteranges media type with one part. A client that cannot
decode a multipart/byteranges message MUST NOT ask for multiple
ranges in a single request.
When a client requests multiple ranges in one request, the server
SHOULD return them in the order that they appeared in the request.
If the server ignores a byte-range-spec because it is syntactically
invalid, the server SHOULD treat the request as if the invalid Range
header field did not exist. (Normally, this means return a 200
response containing the full representation).
If the server receives a request (other than one including an If-
Range header field) with an unsatisfiable Range header field (that
is, all of whose byte-range-spec values have a first-byte-pos value
greater than the current length of the selected resource), it SHOULD
return a response code of 416 (Requested range not satisfiable)
(Section 3.2).
Note: Clients cannot depend on servers to send a 416 (Requested
range not satisfiable) response instead of a 200 (OK) response for
an unsatisfiable Range header field, since not all servers
implement this header field.
5.3. If-Range
If a client has a partial copy of a representation in its cache, and
wishes to have an up-to-date copy of the entire representation in its
cache, it could use the Range header field with a conditional GET
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(using either or both of If-Unmodified-Since and If-Match.) However,
if the condition fails because the representation has been modified,
the client would then have to make a second request to obtain the
entire current representation.
The "If-Range" header field allows a client to "short-circuit" the
second request. Informally, its meaning is "if the representation is
unchanged, send me the part(s) that I am missing; otherwise, send me
the entire new representation".
If-Range = "If-Range" ":" OWS If-Range-v
If-Range-v = entity-tag / HTTP-date
If the client has no entity-tag for a representation, but does have a
Last-Modified date, it MAY use that date in an If-Range header field.
(The server can distinguish between a valid HTTP-date and any form of
entity-tag by examining no more than two characters.) The If-Range
header field SHOULD only be used together with a Range header field,
and MUST be ignored if the request does not include a Range header
field, or if the server does not support the sub-range operation.
If the entity-tag given in the If-Range header field matches the
current cache validator for the representation, then the server
SHOULD provide the specified sub-range of the representation using a
206 (Partial Content) response. If the cache validator does not
match, then the server SHOULD return the entire representation using
a 200 (OK) response.
5.4. Range5.4.1. Byte Ranges
Since all HTTP representations are transferred as sequences of bytes,
the concept of a byte range is meaningful for any HTTP
representation. (However, not all clients and servers need to
support byte-range operations.)
Byte range specifications in HTTP apply to the sequence of bytes in
the representation body (not necessarily the same as the message-
body).
A byte range operation MAY specify a single range of bytes, or a set
of ranges within a single representation.
byte-ranges-specifier = bytes-unit "=" byte-range-set
byte-range-set = 1#( byte-range-spec / suffix-byte-range-spec )
byte-range-spec = first-byte-pos "-" [ last-byte-pos ]
first-byte-pos = 1*DIGIT
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last-byte-pos = 1*DIGIT
The first-byte-pos value in a byte-range-spec gives the byte-offset
of the first byte in a range. The last-byte-pos value gives the
byte-offset of the last byte in the range; that is, the byte
positions specified are inclusive. Byte offsets start at zero.
If the last-byte-pos value is present, it MUST be greater than or
equal to the first-byte-pos in that byte-range-spec, or the byte-
range-spec is syntactically invalid. The recipient of a byte-range-
set that includes one or more syntactically invalid byte-range-spec
values MUST ignore the header field that includes that byte-range-
set.
If the last-byte-pos value is absent, or if the value is greater than
or equal to the current length of the representation body, last-byte-
pos is taken to be equal to one less than the current length of the
representation in bytes.
By its choice of last-byte-pos, a client can limit the number of
bytes retrieved without knowing the size of the representation.
suffix-byte-range-spec = "-" suffix-length
suffix-length = 1*DIGIT
A suffix-byte-range-spec is used to specify the suffix of the
representation body, of a length given by the suffix-length value.
(That is, this form specifies the last N bytes of a representation.)
If the representation is shorter than the specified suffix-length,
the entire representation is used.
If a syntactically valid byte-range-set includes at least one byte-
range-spec whose first-byte-pos is less than the current length of
the representation, or at least one suffix-byte-range-spec with a
non-zero suffix-length, then the byte-range-set is satisfiable.
Otherwise, the byte-range-set is unsatisfiable. If the byte-range-
set is unsatisfiable, the server SHOULD return a response with a 416
(Requested range not satisfiable) status code. Otherwise, the server
SHOULD return a response with a 206 (Partial Content) status code
containing the satisfiable ranges of the representation.
Examples of byte-ranges-specifier values (assuming a representation
of length 10000):
o The first 500 bytes (byte offsets 0-499, inclusive):
bytes=0-499
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o The second 500 bytes (byte offsets 500-999, inclusive):
bytes=500-999
o The final 500 bytes (byte offsets 9500-9999, inclusive):
bytes=-500
Or:
bytes=9500-
o The first and last bytes only (bytes 0 and 9999):
bytes=0-0,-1
o Several legal but not canonical specifications of the second 500
bytes (byte offsets 500-999, inclusive):
bytes=500-600,601-999
bytes=500-700,601-999
5.4.2. Range Retrieval Requests
The "Range" header field defines the GET method (conditional or not)
to request one or more sub-ranges of the response representation
body, instead of the entire representation body.
Range = "Range" ":" OWS Range-v
Range-v = byte-ranges-specifier
/ other-ranges-specifier
other-ranges-specifier = other-range-unit "=" other-range-set
other-range-set = 1*CHAR
A server MAY ignore the Range header field. However, HTTP/1.1 origin
servers and intermediate caches ought to support byte ranges when
possible, since Range supports efficient recovery from partially
failed transfers, and supports efficient partial retrieval of large
representations.
If the server supports the Range header field and the specified range
or ranges are appropriate for the representation:
o The presence of a Range header field in an unconditional GET
modifies what is returned if the GET is otherwise successful. In
other words, the response carries a status code of 206 (Partial
Content) instead of 200 (OK).
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o The presence of a Range header field in a conditional GET (a
request using one or both of If-Modified-Since and If-None-Match,
or one or both of If-Unmodified-Since and If-Match) modifies what
is returned if the GET is otherwise successful and the condition
is true. It does not affect the 304 (Not Modified) response
returned if the conditional is false.
In some cases, it might be more appropriate to use the If-Range
header field (see Section 5.3) in addition to the Range header field.
If a proxy that supports ranges receives a Range request, forwards
the request to an inbound server, and receives an entire
representation in reply, it MAY only return the requested range to
its client.
6. IANA Considerations6.1. Status Code Registration
The HTTP Status Code Registry located at
<http://www.iana.org/assignments/http-status-codes> shall be updated
with the registrations below:
+-------+---------------------------------+-------------+
| Value | Description | Reference |
+-------+---------------------------------+-------------+
| 206 | Partial Content | Section 3.1 |
| 416 | Requested Range Not Satisfiable | Section 3.2 |
+-------+---------------------------------+-------------+
6.2. Header Field Registration
The Message Header Field Registry located at <http://www.iana.org/assignments/message-headers/message-header-index.html> shall be
updated with the permanent registrations below (see [RFC3864]):
+-------------------+----------+----------+-------------+
| Header Field Name | Protocol | Status | Reference |
+-------------------+----------+----------+-------------+
| Accept-Ranges | http | standard | Section 5.1 |
| Content-Range | http | standard | Section 5.2 |
| If-Range | http | standard | Section 5.3 |
| Range | http | standard | Section 5.4 |
+-------------------+----------+----------+-------------+
The change controller is: "IETF (iesg@ietf.org) - Internet
Engineering Task Force".
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